A broad-spectrum gas sensor based on correlated two-dimensional electron gas

Designing a broad-spectrum gas sensor capable of identifying gas components in complex environments, such as mixed atmospheres or extreme temperatures, is a significant concern for various technologies, including energy, geological science, and planetary exploration. The main challenge lies in finding materials that exhibit high chemical stability and wide working temperature range. Materials that amplify signals through non-chemical methods could open up new sensing avenues. Here, we present the discovery of a broad-spectrum gas sensor utilizing correlated two-dimensional electron gas at a delta-doped LaAlO3/SrTiO3 interface with LaFeO3. Our study reveals that a back-gating on this two-dimensional electron gas can induce a non-volatile metal to insulator transition, which consequently can activate the two-dimensional electron gas to sensitively and quantitatively probe very broad gas species, no matter whether they are polar, non-polar, or inert gases. Different gas species cause resistance change at their sublimation or boiling temperature and a well-defined phase transition angle can quantitatively determine their partial pressures. Such unique correlated two-dimensional electron gas sensor is not affected by gas mixtures and maintains a wide operating temperature range. Furthermore, its readout is a simple measurement of electric resistance change, thus providing a very low-cost and high-efficient broad-spectrum sensing technique.


Growth of LaFeO3 and LaAlO3 films
The LaFeO3 (LFO) and LaAlO3 (LAO) films were grown by pulsed laser deposition (PLD) in a layer by layer fashion as shown in Fig. S1a.The thickness of the LFO and LAO was controlled by counting the RHEED oscillations, enabling a precise unit cell (u.c.) control of the growth.RHEED patterns (See Fig. S1, b-c) indicate a 2D smooth surface.The AFM surface topography (Fig. S1d) and measured step heights (Fig. S1e) before growth indicate that a high-quality single TiO2-terminated SrTiO3 (001) substrates were obtained.Highly smooth film AFM surface image (Fig. S1f) shows that the grown film still has a two-dimensional structure, and measured step heights (Fig. S1g) of thin films shows that the LAO is subjected to tensile strain.X-ray diffraction (Fig. S2a) shows that the heterojunction film has a good epitaxial relationship with the STO substrate.The full-width at half-maximum (FWHM, Fig. S2b) was measured to be 0.021°, indicating the good crystallinity of the heterojunction film.

X-ray diffraction and reflectivity of heterojunction film
The simulation of XRR (Fig. S2c, red line) well confirm the layer by layer fashion monitored by RHEED.The simulation parameters are shown in Table S1.Since the capped LAO layer blocks the release of excited electrons, the electron yield decreases, leading to a reduction of total electron yield signal and thus intensity in the underneath Fe XAS.However, the XAS results do not show an evident change of valence state of Fe.By adjusting the carrier concentration at the interface of C-2DEG gas sensor, the resistance after activation does not exceed the measurement limit of the source meter, so as to realize the detection of those substances that sublime or boil at low temperature, such as H2 (Fig. S3a) and Ar (Fig. S3b).Two smooth curves (Fig. S4a) show that applying back-gating at high temperature (300K) does not activate the device, but only the gas ice layer can be charged.The sudden change of the slope of the first differential (Fig. S4b) indicates the sublimation of CO2.Based on the parallel plate series model (1/C=1/C1+1/C2), the total capacitance is mainly contributed by the substrate since the film thickness is much thinner than the substrate.Under the temperature of 2 K and the electric field of 4200 V/cm, the STO (001) substrate has a relative permittivity of ~10 4 . 1 Therefore, after applying a backgate voltage of 210 V, the gas charge concentration is ~2.32×10 13 /cm 2 , which is consistent with the depleted interface carrier concentration calculated in the main text.The carrier mobility after activation is almost the same as that of as grown, except that there is a maximum 20% change in the SOB temperature range, indicating that the activation does not change the carrier but only reduces the carrier concentration.

Fig. S6 | Effect of devices with different carrier concentrations on temperature.
The circular markers correspond to the sensor with a carrier concentration of 1.30 × 10 13 /cm 2 , while the square markers correspond to the sensor with a carrier concentration of 2.54 × 10 13 /cm 2 .The temperature difference associated with both the inflection points and the minimum value of the first-order differential of the two sensor resistances is less than 0.5 K within the error bars.

Layer
Chem.Formula Density (g/cm  Table S3.A literature review on the electron affinity of all gases appearing in the article.The higher the electron affinity, the stronger the ability to bind electrons, the more sensitive our gas sensors are to this gas and thus have a higher measurement limit.

Fig. S1 |
Fig. S1 | Growth and surface characterization.a, RHEED intensity oscillations during the growth of LFO (5 u.c.) and LAO (10 u.c.).The growth started at t = 0 and stopped at the time indicated by arrows.RHEED patterns before (b) and post (c) growth.AFM surface topography before (d) and post (f) growth.Measured step heights before (e) and post (g) growth.

Fig. S2 |
Fig. S2 | Structure characterizations.a, X-ray diffraction of (001) and (002) peaks for the heterojunction film.b, X-ray diffraction ω-rocking curve of the as-grown heterojunction film (002) plane.Experimental (black dots) and model (red line) X-ray reflectivity curves (c) of the heterojunction film.d, X-ray scattering length density (SLD) profiles obtained from the X-ray reflectivity fits.

Fig. S5 |
Fig. S5 | Evidences for Charged gas ice layer.A, R-T curves of cooling (red line) and warmup (blue line) with gate applied at 300K.b, Response of sample with higher carrier concentration (2.54×10 13 /cm 2 ) in mixed gas environment.The inset is the first order differential of resistance around the CO2 sublimation temperature (135 K -150 K).

Fig. S6 |
Fig. S6 | Carrier mobility before (orange line) and after (blue line) activation as a function of temperature.

Table S2 . A literature review on the performance parameters and conditions of CO2 gas sensor.
According to the parallel plate capacitor model, the device exhibits an instantaneous response when a substance undergoes sublimation or boiling.which is manifested as a change in the slope of the C-2DEG resistance at the interface.To precisely characterize gas partial pressure, data should be acquired within a range of ±10 K around the sublimation or boiling point.Consequently, the response time for measuring partial pressure depends on the heating rate (2 -40 K/min).As a result, the response time can range anywhere from 30 to 600 s.